In oil wells, natural gas wells, and the like (hereinafter also collectively referred to as “oil wells”), steel pipes referred to as oil country tubular goods (OCTG) such as casings and tubings are used for extraction of underground resources. The steel pipes are sequentially connected to each other, and threaded joints are used for the connection.
Threaded joints for steel pipes are classified into two types: coupling-type joints and integral-type joints. A coupling-type threaded joint is constituted by a pair of tubular goods that are to be connected to each other, of which one is a steel pipe and the other is a coupling. In this case, the steel pipe includes male threaded portions formed on the outer peripheries at both ends thereof, and the coupling includes female threaded portions formed on the inner peripheries at both ends thereof. Then, the steel pipe and the coupling are connected to each other. An integral-type threaded joint is constituted by a pair of steel pipes as tubular goods that are to be connected to each other, without a separate coupling being used. In this case, each steel pipe includes a male threaded portion formed on the outer periphery at one end thereof and a female threaded portion formed on the inner periphery at the other end thereof. Then, the one steel pipe and the other steel pipe are connected to each other.
In general, the joint portion at the tubular end where a male threaded portion is disposed is referred to as a pin because it includes an element that is inserted into a female threaded portion. On the other hand, the joint portion at the tubular end where a female threaded portion is disposed is referred to as a box because it includes an element that receives a male threaded portion. Pins and boxes both have a tubular shape because they are constituted by end portions of tubular goods.
In recent years, oil wells have increasingly become deep-underground or ultra-deep-water wells and accordingly oil well environments have become harsh with high temperatures, high pressures, and high corrosivity. To deal with such harsh environments, heavy wall steel pipes are widely used as oil country tubular goods. A threaded joint for connecting such steel pipes is required to provide excellent sealing performance against pressure from the internal thereof (hereinafter also referred to as “internal pressure”) and pressure from the external thereof (hereinafter also referred to as “external pressure”).
The following is a conventional technique for the improvement of sealing performance for threaded joints of heavy wall steel pipes. International Publication No. WO01/029476 (Patent Literature 1) discloses a threaded joint having two surface-to-surface seal portions. In the technique of Patent Literature 1, a pin includes, in order from a free end of the pin toward a tubular body, a shoulder surface, a first sealing surface, a first male threaded portion, a second sealing surface, and a second male threaded portion. A box includes a shoulder surface, a first sealing surface, a first female threaded portion, a second sealing surface, and a second female threaded portion, which correspond to the above portions of the pin, respectively. A first threaded portion constructed of the first male threaded portion and the first female threaded portion is a tapered threaded portion with a trapezoidal thread. The same is true for a second threaded portion constructed of the second male threaded portion and the second female threaded portion.
The first threaded portion and the second threaded portion engage in intimate contact with each other in a fastened state and have an interference fit. Both the first sealing surfaces and the second sealing surfaces are respectively brought into contact with each other by the screwing of the pin, and in a fastened state, they engage in intimate contact with each other and have an interference fit. Both the shoulder surfaces are brought into contact with each other by the screwing of the pin, and serve as stopper for restricting the screwing of the pin. The shoulder surfaces serve as a stopper, and in a fastened state, serve to impart so-called thread tightening axial force to the load flanks of the respective pins of the first threaded portion and the second threaded portion.
In the case of the first threaded portion and the second threaded portion disclosed in Patent Literature 1, in a fastened state, both the load flanks of the pin and the box are in contact with each other, the root of the pin and the crest of the box are in contact with each other, and further, the crest of the pin and the root of the box are in contact with each other.
With a threaded joint of Patent Literature 1 having this configuration, a firm threaded connection is ensured because of the engagement and intimate contact between the first threaded portion and the second threaded portion. In addition to this, by the engagement and intimate contact between the first sealing surfaces, a surface-to-surface seal portion closer to the internal is formed, and a sealing performance against internal pressure is mainly ensured. In addition, by the engagement and intimate contact between the second sealing surfaces, a surface-to-surface seal portion closer to the external is formed, and a sealing performance against external pressure is mainly ensured.